Images printed on a press are printed using a halftone screen. The screen uses round dots of varying sizes to approximate light colors. To make a graphic appear 25% gray, small black dot are arranged in a grid so that 25% of the paper is covered in black ink and 75% of the paper is not covered in black ink. To the eye, the area appears gray. Take a close look at a black and white photo in the newspaper, you'll see the halftone dots.
Printing is an imperfect process and as the wet ink touches the paper, it is absorbed and spreads out. That makes each of those black dots bigger which makes the overall image darker.
The solution to this problem is to lighten your image by about the same amount as you expect the ink to gain. We deliberately make our images and graphics too light so they'll darken up on the press to look like what we really want.
Our printer has an average black gain of about 15% so you'll want to make the darkest "black" in your image about 85% gray.
a continuous tone image adjusted to look good onscreen
same image converted to a halftone image
Photoshop uses additive color values. To lighten your black point from 100% to 85% you use the value of 38. A Photoshop Conversion Table can be found at www.esolutions.com
Images and graphics adjusted for dot gain will look too light on your screen but will print correctly on press.
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